Inside the dungeon walk east to the bridge. There will be a conversation between Zeith, Vrid and Reaper Yanku.

You need to kill all monsters in the zone to be able to move to the second zone. An announcement will show if all the monster has been killed. On the second zone, you also need to kill all monsters to be able to move to final zone.

On the final zone, head to the center of the room where you will see Zeith and Vrid. Reaper Yanku will appear and froze them (you will be frozen also, Evil Druid card or unfrozen armor works here).

There will be a wave of Orc Zombies, followed by Verit and Megalodon that you need to kill.

Once all undead wave has been cleared, Reaper Yanku with 50m HP will appear and you can kill him now. Once he's dead, he will drop Prize of Hero at 100% chance.

Once done, talk to Vrid again to receive Token of Hero as well as move you out from the instance.